1. Field of the Invention
The present invention relates to a marine vessel propulsion device in which the power of an engine is transmitted to a propeller shaft through a drive shaft and a dog clutch. Examples of such a marine vessel propulsion device include an outboard motor and astern drive.
2. Description of the Related Art
An outboard motor according to a conventional technique is disclosed by Japanese Unexamined Patent Application Publication No. 2006-183694. This outboard motor includes an engine, a drive shaft whose upper end is connected to a crankshaft of the engine, a forward/backward movement switching mechanism connected to a lower end of the drive shaft, a propeller shaft connected to the forward/backward movement switching mechanism, and a propeller attached to the propeller shaft. The forward/backward movement switching mechanism arranges a dog clutch that includes a pair of bevel gears that rotate in mutually opposite directions by means of power transmitted from the drive shaft and a dog gear that is splined to the propeller shaft. A driving force is transmitted to the propeller shaft by allowing the dog gear to engage with either of the pair of bevel gears. A damper structure that reduces the occurrence of an abnormal noise caused by a torque variation of the engine is disposed at a place between both ends of the drive shaft. Relative rotation between the bevel gear and the dog gear in the dog clutch is caused by the torque variation of the engine, and these are brought into contact with each other or are separated from each other. A gear rattle is repeatedly caused by shocks caused when coming into contact therewith, and a so-called rattling noise is caused. This gear rattle is deadened by the damper structure. The rotational fluctuation of the engine is liable to occur when the engine rotation speed is low (especially when idling). Moreover, an engine sound is low when the engine rotation speed is low, and therefore the gear rattle is conspicuously easily heard.